Eperimental studies on the structure of the lower mantle and themantle fractionation

在Mg_2SiO_4-Fe_2SiO_4体系中，测定了橄榄石（）-改性尖晶石（）-尖晶石(y)-后尖晶石相平衡。结果表明，镁尖晶石（Fe/Mg+Fe<0.2）在极窄的压力区间内（1600℃时< 0.15 GPA）解离成钙钛矿和镁辉石矿。解离压力几乎与铁含量无关，与650km处的解离压力一致。这些结果表明，650km不连续的尖锐性确实可能是由于橄榄岩地幔的这种分离所致。考虑到CaSiO_3-MgSiO_3-Al_2O_3体系中证实的主晶石解离作用和地幔橄榄岩的解离作用，将尖晶石解离作用的相界设定为反射波所暗示的尖锐不连续，估计过渡带底部（65km深度）的温度约为1600℃，建立了下地幔的矿物学模型。在2000 ~ 3000℃的温度下，在2.5 GPa的温度下，对一肥沃的橄榄岩进行了熔融实验，发现富mgsio_3的钙钛矿在下地幔条件下为液态相。以Mg/Si， Al/Si， Ca/Si和Ca/Al比值为约束条件，表明原始上地幔成分可以通过少量mgsio_3 -钙钛矿分选（10-20wt%）从球粒陨石来源获得。用单晶法测定了mgsio_3钙钛矿在9.6GPa高压和常压下的晶体结构。其结构特征为SiO_6八面体与Mg在八重配位立方体内的三维框架，胞元压缩主要由SiO_6八面体的倾斜控制。首次利用布里渊光谱在常压和室温条件下测定了MgSiO_3钙钛矿的单晶弹性模。我们的弹性数据与下地幔地震模型的比较表明，只有当MgSiO_3钙钛矿的剪切模量具有很强的负温度导数时，具有软锰矿或辉石化学计量的岩石学模型才能与实验数据相容。少

The olivine( )-modified spinel( )-spinel(y)-postspinel phase equilibria in the system Mg_2SiO_4-Fe_2SiO_4 were determined up to 26 GPA and 1600 C. It was demonstrated that the magenesian spinel (with Fe/Mg+Fe<0.2) dissociates into perovskite and magnesiowustite within an extremely narrow pressure interval (<0,15 GPa at 1600 C). The dissociation pressure is almost independent of iron content and coincide to that at 650km. These results indicate that the sharpness of the 650km discontinuity may indeed be due to this dissociation in peridotitic mantle. Assigning the phase boundaries of the spinel dissociation to the sharp discontinuity implied by study of reflected waves, the temperature at the base of the transition zone (655km depth) is estimated to be about 1600 C. Taking the dissociation of the majorite dissociation confirmed in the system CaSiO_3-MgSiO_3-Al_2O_3 and mantle peridotites into consideration, the mineralogical model was constructed down to the lower mantle.Melting experim … More ents on a fertile peridotite was performed up to 2.5 GPa and at temperatures 2000-3000 C. The MgSiO_3-rich perovskite was found to be the liquidus phase at the lower mantle conditions. Using the Mg/Si, Al/Si, Ca/Si, and Ca/Al ratio as constraints, it was shown that primitive upper mantle compositions can be derived from a chondritic source through a small degree of MgSiO_3-perovskite fractionation (10-20wt%).Crystal structure of MgSiO_3perovskite was determined at high pressures up to 9.6GPa as well as at atmospheric pressure, using single crystals. The structure was characterized by the three dimensional framework of SiO_6 octahedra with Mg in the eight-fold coordinated cuboctahedron and the unit cell compression is controlled dominantly by the tilting of SiO_6 octahedra. The singly-crystal elastic module of MgSiO_3 perovskite was determined for the first time under atmospheric pressure and room temperature using Brillouin spectroscopy. Comparison of our elasticity data with seismic models appropriste to the lower mantle reveals that petrological models having either pyrolite or pyroxene stoichiometries are compatible with the experimental data only if the shear modulus of MgSiO_3 perovskite has a very strong negative temperature derivative. Less